Lithium ion secondary batteries have high energy density and excellent charge/discharge cycle characteristics, and are thus widely used for a power supply for compact mobile devices such as cellular phones and laptop computers. In addition, the recent increasing environmental considerations and growing consciousness of energy saving have been promoting a demand for large batteries having a large capacity and a long life in the fields of electric vehicles, hybrid electric vehicles, power storage, etc.
In general, a lithium ion secondary battery primarily consists of: a negative electrode including a negative electrode active material of a carbon material capable of intercalating and deintercalating a lithium ion; a positive electrode including a positive electrode active material of a lithium composite oxide capable of intercalating and deintercalating a lithium ion; a separator separating the negative electrode and the positive electrode; and a non-aqueous electrolytic solution prepared by dissolving a lithium salt in a non-aqueous solvent.
Amorphous carbon or graphite is used for the carbon material used as the negative electrode active material, and graphite is typically used particularly in an application which requires a high energy density. And thus, development has been conducted for various graphite-based materials.
For example, Patent Literature 1 discloses a composite graphite particle as a negative electrode material for a lithium ion battery, wherein the composite graphite particle includes a core material containing graphite and a carbonaceous layer present on the surface of the core material. Patent Literature 1 also discloses the followings: this carbonaceous layer is obtained by heat-treating the core material which has been attached with an organic compound at a temperature of 500° C. or higher; the content of the carbonaceous layer is 0.05 to 10 parts by mass based on 100 parts by mass of the core material; and the BET specific surface area is 0.2 to 30 m2/g. Patent Literature 1 further discloses that the composite graphite particle has a high acceptability for a lithium ion, and that therefore, a lithium ion battery having good cycle characteristics and output/input characteristics can be obtained by using the composite graphite particle.
Patent Literature 2 disclose use of a negative electrode for a non-aqueous electrolyte secondary battery, wherein the negative electrode includes a graphite particle including a mixture of a covered graphite particle covered with amorphous carbon and a non-covered graphite particle not covered with amorphous carbon. Patent Literature 2 also discloses that the mass fraction of amorphous carbon to 100 parts by mass of the covered graphite particle is 0.1 to 10 parts by mass, and that the specific surface area of the graphite particle mixture of the covered graphite particle and the non-covered graphite particle is 2 to 5 m2/g. Patent Literature 2 further discloses that use of the graphite particle mixture enables inhibition of the precipitation of lithium in charging at a high rate, and enables suppression of cycle degradation through lowering the internal resistance of a negative electrode.
Patent Literature 3 discloses a mixed carbon material including a carbon material A and a carbon material B, wherein the carbon material A and the carbon material B each include a core material including a graphite powder and a surface carbon material (at least one of amorphous carbon and turbostratic carbon) covering or attaching to a part of the surface. Patent Literature 3 also discloses the followings: the compression density of the carbon material A is 1.80 to 1.90 g/cm3; the compression density of the carbon material B is 1.45 to 1.65 g/cm3; the compression density of the mixed carbon material is 1.75 to 1.84 g/cm3; the average particle diameter of the carbon material B is 7 μm or larger and 14 μm or smaller and smaller than the average particle diameter of the carbon material A; the specific surface area of the carbon material A is 4 m2/g or smaller; and the specific surface area of the carbon material B is 6 m2/g or smaller. Patent Literature 3 further discloses that a lithium ion secondary battery with the mixed carbon material can achieve high charge acceptability in combination with a high capacity due to high negative electrode density, and in addition has less irreversible capacity.
From another viewpoint, Patent Literature 4 discloses the followings: when a negative electrode is pressed to increase the packing density in order to enhance the capacity of the negative electrode, a portion near the surface of the negative electrode is excessively compressed, and the size of a void present near the surface becomes smaller than that in the interior portion to interrupt the permeation of a non-aqueous electrolytic solution into the negative electrode, which causes shortage of the amount of the electrolytic solution retained in the negative electrode to thereby lead to the degradation of the charge/discharge cycle characteristics of a secondary battery. To solve the problems, a negative electrode is fabricated in Patent Literature 4 by using a method including a step of forming an active material-containing layer having a density of 1.0 to 1.3 g/cm3 by coating a current collector with a coating material containing a fibrous carbonaceous material and a graphite material followed by drying, and a step of pressing the active material-containing layer to increase the density thereof to 1.3 to 1.6 g/cm3. Patent Literature 4 discloses that fabrication of a negative electrode in this manner enables homogenization of the sizes of voids in the active material-containing layer of the negative electrode, and that thus the charge/discharge cycle life of a secondary battery can be enhanced.
Patent Literature 5 discloses a carbonaceous electrode plate for a non-aqueous electrolytic solution secondary battery, wherein the carbonaceous electrode plate includes a sheet-shaped carbonaceous molded product having a density distribution or a porosity distribution in the thickness direction of the sheet and the interior portion of the carbonaceous electrode plate has a higher density or a lower porosity than the outer surface portion. Patent Literature 5 also discloses the followings: the outer surface portion functions as a negative electrode and simultaneously works to allow an electrolytic solution to permeate into the interior portion of the electrode; the interior layer serves to dope/dedope more lithium and simultaneously functions as a current collector having a high electroconductivity; and thus the carbonaceous electrode plate is an excellent negative electrode.